The present invention relates to a high-frequency oscillation circuit, and more particularly, to a high-frequency oscillation circuit useful for enhancing sensitivity of various measuring instruments used as weight sensor, chemical sensor, biosensor, viscosity sensor, film thickness meter, gas sensor, floating dust sensor, immunity sensor or the like.
While recently various measuring instruments using crystal as weight sensor, chemical sensor, biosensor, viscosity sensor, film thickness meter or the like have been numerously developed, it has been needed urgently to develop a high precision and highly sensitive measuring instruments to cope with such a demand for diversity of materials to be detected and precise quantitative determination of materials to be detected.
As is generally known, however, a wafer used for a crystal resonator has such a nature as to cause distortion (piezo-electric effect) when a voltage is applied to thin film electrodes attached to both side faces faces thereof and return to its initial state when the voltage is removed. Because of this nature, a crystal resonator oscillates at a natural frequency determined by its thickness. Thereby, in a crystal wafer, when its thickness varies by adsorbing a substance, a basic frequency (i.e., the natural oscillation frequency or basic oscillation frequency) of the crystal resonator is varied.
The change xcex94f of this natural oscillation frequency is proportionate to a change in thickness. If the change in thickness dimension is replaced by a change xcex94m in mass, the following equation called Sauerbrey""s equation can be introduced.
xcex94f=xe2x88x92{2f02/(xcfx81qxc3x97xcexcg)1/2}xc3x97(xcex94m/A)
wherein f0 is a basic oscillation frequency, xcfx81g and xcexcg are density and elastic modulus of the crystal, respectively, and A is the area of a portion performing piezo-electric response.
From this equation, it is understood that, since the sensitivity xcex94f is proportionate to the square of the basic oscillation frequency f0, it is desirable to use a crystal resonator whose f0 is great. However, if f0 becomes too great, the thickness is reduced, and the oscillator tends to be easily broken. Therefore, it is general to use a crystal resonator whose f0 is between 5 and 10 MHz under the normal atmosphere, and even in a solution, merely a crystal resonator whose greatest oscillation frequency f0 is 30 MHz is used, and a measurement exceeding the maximum detection limit of a general-purpose crystal resonator has not yet been attained.
While there is also an example of measurement using the seventh order overtone mode (63 MHz) of a crystal resonator whose f0 is 9 MHz, its detection limit is reported as 0.1 ng, which showed no remarkable improvement in sensitivity, compared with the conventional method of 1 ng (xe2x80x9cThe Latest Method of Separation, Purification and Detectionxe2x80x9d, p. 441, by NTS Publishing Co., issued May 26, 1997).
On the other hand, in contrast with such a situation as above, there is also proposed a high-frequency oscillation circuit using a crystal resonator not as a weight sensor but for controlling the frequency of an oscillation circuit.
However, in many cases these circuits were analog circuits which are complicated and hard-to-adjust as numerous parts such as transistor, coupling transformer, inductance, etc., are used, and were expensive and not suitable to be used as a measuring instrument for various sensors.
A low-frequency oscillation circuits using a logic element in part is known (JP-A-3-165236 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application), xe2x80x9cThe Electronic Circuit Parts Utilization Handbookxe2x80x9d, p.67, by CQ Publishing Co., issued on Nov. 1, 1985). This oscillation circuits, however, use only an oscillator having a natural frequency in a low-frequency area, so that it cannot cope with a demand for higher sensitivity, making it hard to realize a high-frequency oscillation circuit showing high-frequency stability. Furthermore, there was a need to design and constitute an oscillation circuit to suit a crystal resonator to be oscillated and its frequency (Japanese patent application No. 2000-31513).
The present invention overturns common knowledge of a conventional oscillation circuit.
The present invention aims to provide a high-frequency oscillation circuit which can keep a stable high-frequency oscillation to easily cope with the natural oscillation frequency of a crystal resonator as a sensor even if it becomes high, and yet which can be easily manufactured at a low cost.
Other and further objects, features, and advantages of the invention will appear more fully from the following description, take in connection with the accompanying drawings.